Solenoids are generally known and used for a variety of purposes. In some applications it is useful to have a solenoid that provides a relatively constant force over a relatively long stroke. This type of solenoid, commonly called a linear solenoid, uses a variable overlap in the working air gap generally associated with an armature to generate an electromagnetic force in the direction of the solenoid axis extending along the longitudinal length of the armature. Undesirable eccentricity of the armature is an inherent problem with solenoids. Conventional solenoids have two air gaps disposed axially along the armature so that eccentricity of the armature causes both air gaps to be reduced. Any eccentricity of the armature will cause uneven distribution of magnetic flux and will result in an undesirable radial force acting perpendicular to the solenoid axis. Manufacturing imperfections in the solenoid components, clearance with the bearings associated with the armature, assembly of the solenoid components in less than perfect alignment, and the like can all contribute to eccentricity.
Typically, the force generated in the air gap of a solenoid acts to move the armature in a direction that will reduce the reluctance of the air gap. The reluctance of the air gap in a magnetic circuit is proportional to the area of the air gap and inversely proportional to the distance of the gap. As such, an eccentric armature will be more strongly attracted toward the nearer side of the pole piece of the solenoid. Thus, an increased radial force acting on the armature will be applied to any associated component surfaces, e.g., between an armature pin and bearing surfaces, resulting in friction between components. Friction with the components degrades the performance of the solenoid and causes wear.
Accordingly, there exists a need for an improved solenoid arrangement that helps to minimize the radial force due to eccentricity while substantially preserving the level of axial force.